Some 4,000 people and 12 marching bands from across Ireland, North America and Austria took part in the parade this year.
While the tradition of St. Patrick's Day parades first began among Irish immigrant communities in the United States in the 1700s, and the biggest parade these days takes place in New York City, officials say the holiday draws some 100,000 tourists to Ireland each year.
Smaller parades also took place in cities from Limerick to Cork and Belfast in Northern Ireland.
In London, Kate, the Princess of Wales, sported a forest green long coat with matching hat as she marked the occasion with a solo visit to the Irish Guards at Wellington Barracks.
The royal, who is the ceremonial colonel of the regiment, presented fresh shamrock sprigs and awarded medals to soldiers before the regiment's St. Patrick's Day parade. She enjoyed a half pint of Guinness afterward as she chatted and joked with the soldiers.
Kate, who missed the Guards' celebrations last year after she was diagnosed with cancer, has gradually returned to public duties.
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP
Credit: AP